Normal map on mirrored model
It's a great shame 128 isn't half of 255.

This isn't an answer to your question, but it does what you do to fix it without having to construct a second map as a mask.



This node tree compares the map-encoded normal to (0,0,1), and if it's close enough, sets it to (0,0,1)



How close 'close enough' is, is set in the 'Less Than' node. The color in the 'Color' node is RGBA 0,0,1,0. (Is there a better way of entering a fixed vector?)